Recently, the social environment surrounding us has been changing greatly according to an improvement in information communication network. Among such changes, the growth of mobile equipments is included. Their markets are being extended according to compactness and high functionality thereof. For this reason, further compactness of semiconductor package and multi-layer wiring substrate which is capable of mounting the package with high density are required. Further, inter-layer connection enabling high density wiring, i.e., technique for forming multiple layers with high density becomes important.
A through-hole connection that drilling and plating processes are combined is mainly provided as the method of forming plurality layers and widely known. According to this method, however, since holes are formed at all layers, a wiring capacity is limited.
In order to reduce the volume of holes at connecting portions, a build-up technique that processes of forming an insulating resin composition layer, drilling and forming a circuit are repeated has been mainly utilized. The build-up technique is roughly classified into a laser method and a photolithography method. According to the laser method, laser irradiation is performed in order to drill an insulating resin composition layer. On the other hand, according to the photolithography method, a photosensitive curing agent (photoinitiator) is used for insulating resin composition layer, a photo mask is placed thereon, exposure and development are performed for the layers and thus holes are formed.
Some inter-layer connection methods are proposed in order to obtain further reduction in costs and high density. Among such methods, a method capable of omitting processes of drilling and plating a conductive layer has attracted attention. According to this method, a bump is formed on a wiring of substrate by printing a conductive paste. Then, an inter-layer connection insulating material in a state of B stage and a metallic layer are disposed, the bump is inserted within a mold resin by pressing so as to be conductive connected to the metallic layer. This method that the bump is inserted is published in academic society and newspaper, and widely recognized in a field of printed wiring board (Hiroshi Ohira and two other persons, Suggestion of Printed Wiring Board by New Manufacturing Method, A Collection of Papers at the Ninth Lecture Meeting Regarding Circuit Mount, ISSN 0916-0043, 15A-10, PP. 55-56, (1995.3.14-16), Takahiro Mori and five other persons, Application and Miniaturization of Substrate Using Inter-Layer Connection Technique Using Bump, A Collection of Papers at the Tenth Lecture Meeting Regarding Circuit Mount, ISSN 0916-0043, 15A-09, pp. 79-80, (1996.3.13-15).
A device that plated wires are embedded into an elastomer such as silicon rubber in its thickness direction is developed and utilized as a convenient tool for connecting two conductors.
According to the laser method of the prior arts, a selection range of insulating materials is large, although drilling can be performed not only for adjacent layers but also for adjacent layers and next adjacent layers, there arise problems in that a de-smear treatment is required for removing foreign matters of resin diffused by laser irradiation and a working cost is increased in proportion to the number of holes.
According to the photolithography method, conventional facilities for manufacturing wiring board can be utilized and drilling can be performed at a time and thus a reduction in costs can be advantageously accomplished. Nevertheless, there arise problems in that the resolution of inter-layer insulating material and heat resistance, moreover strength of adhesion between a circuit and an insulating resin composition layer cannot be accomplished at the same time.
Further, since the bump is formed by printing a conductive paste or plating a metal, the precision of forming the bump relies on the limit of print technology. If the bump is formed by plating, there arises a problem that it is difficult to suppress height variation of bumps especially when diameters of holes are different. Further, the bump formed of conductive paste has small mechanical strength and may be broken by pressing pressure. The connection reliability may be decreased when a drilling process is required.
The method in which plated wires are embedded into an elastomer such as silicon rubber such that two layers of conductors are connected with each other is simple, but it is difficult to embed the wires only into desired connection positions. If the wires are embedded in a lattice configuration, the wires may become obstacles at positions that the wires are not desirably contacted.